I can't answer as to how long it would take for the guppy's fin to grow back, I've never kept them.
However, unless the wound is severe; many fish are able to regrow fins fairly quickly as long as their water is kept pristine.
With bettas in communities it is always a gamble. Things can go sour even in an established situation; however you just added a new "invader" (in his eyes) into his territory. So now he might be back to trying to defend it again.
You can try adding more decor, and rearranging what you have in there; but in all honesty I can't feel comfortable recommending anything else but giving him his own tank. That being said, in the end it is your decision as to how you want to proceed.